The present invention relates to an anisotropic membrane having improved heat resistance and hydrolytic stability. More particularly the present invention relates to such an anisotropic membrane which is usefully employed in separation processes such as ultrafiltration and reverse osmosis. In U.S. Pat. No. 3,567,810 anisotropic membranes (also referred to as asymmetric membranes) are prepared by forming a polymer solution into a membrane and contacting the membrane with a liquid diluent for the solvent which is of a sufficiently low degree of compatibility with the polymer to effect precipitation of the polymer thereby creating an anisotropic structure therein. For the teaching contained therein, U.S. Pat. No. 3,567,810 is hereby incorporated in its entirety by reference thereto.
Polystyrene exists in three distinct molecular forms: isotactic; syndiotactic and atactic. Although having desirable chemical properties, atactic polystyrene has generally failed to result in a suitable asymmetric membrane due to its solvent sensitivity and poor thermal properties. Isotactic polystyrene is known to possess improved resistance to temperatures and better solvent resistance compared to atactic polystyrene. However, the material is relatively slow to form a crystal structure, a necessary step in preparing a membrane. Thus, isotactic polystyrene also cannot be employed to prepare membranes. Syndiotactic polystyrene possesses many of the beneficial attributes of isotactic polystyrene, however, techniques for forming asymmetric membranes of crystalline syndiotactic polystyrene have been previously unknown. The fundamental cause of this failure is that solvents capable of dissolving the polymers at an elevated temperature and also preventing crystallization from taking place rapidly at a reduced temperature have been previously unknown. Crystal formation at ambient conditions such as are desirably utilized to form membranes results in an intractable gel or paste lacking sufficient fluidity for membrane formation.
Syndiotactic vinylaromatic polymers and a process for their preparation have been disclosed in U.S. Pat. No. 4,680,353, the teachings of which are incorporated herein by reference thereto. In the reference a polymerization catalyst is disclosed which is suitably employed in the preparation of vinylaromatic polymers having a syndiotactic configuration. By the term syndiotactic as used herein is meant that the degree of syndiotacticity at a racemic diad in the nuclear magnetic residence (NMR) spectrum is higher for such polymers than for polymers of identical monomers obtained by conventional free radical polymerization techniques. A preferred syndiotactic vinylaromatic polymer is syndiotactic polystyrene having a methyl ethyl keytone insoluable polymer content of at least about 75 weight percent.
Syndiotactic vinylaromatic polymers have significantly enhanced resistance to the effects of heat. That is, the polymers possess elevated melting temperatures. For example, the crystalline melting point of syndiotactic polystyrene is approximately 270.degree. C.
Syndiotactic polyvinylaromatic polymers also possess superior mechanical and chemical properties, such as solvent resistance, compared to atactic polyvinylaromatic polymers. At normal room temperatures syndiotactic polyvinylaromatic polymers are extremely resistant to attack by common chemical solvents. Moreover the polymers possess good hydrolytic stability thus making them well suited for utilization in extreme chemical environments.
However, because of the chemical resistance of syndiotactic polyvinylaromatic polymers, and the other previously mentioned disadvantages, methods for the preparation of membranes therefrom have been wanting. It would be desirable if there were provided a practical method for forming anisotropic membranes of crystalline syndiotactic vinylaromatic polymers.